This invention relates to ultrasonic inspection methods and devices for performing such inspections, and particularly, to ultrasonic inspection methods and devices for inspecting turbine and generator rotors from the rotor bore surface.
Ultrasonic inspections are performed by directing ultrasonic signals into the mass of an object using a suitable transducer. Flaws in the material such as cracks, voids, or inclusions represent acoustic discontinuities that reflect a portion of the incident signal energy. The reflected portion of the signal is in some cases directed back to and may be detected by the transducer that generated the original signal. In other cases, some of the reflected energy may be directed off at an angle from the incident signal to a separate catch transducer capable of detecting the reflected signal. Utilizing the transmitting transducer as the detecting or pick-up transducer is commonly referred to as a pulse/echo inspection. Utilizing separate transmitting and receiving transducers is commonly referred to as a pitch/catch inspection.
Each transducer commonly comprises a piezoelectric element and is mounted in a block of suitable material to form a search unit. Upon receipt of a suitable voltage spike, each piezoelectric element transmits an ultrasonic signal into a material with which the search unit is in intimate accoustical contact. Conversely, upon receipt of a suitable ultrasonic signal from the material, each piezoelectric element produces a voltage signal proportional to the pressure amplitude of the ultrasonic signal incident on the element. The amplitude and shape of the voltage signal produced upon receipt of an ultrasonic signal reflected from a particular acoustic discontinuity (e.g. a flaw) provides information about the discontinuity. Also, the delay time of the returned signal relative to the transmitted signal, when coupled with information concerning the position and orientation of the search unit with respect to the part under inspection and information concerning the central rays or beams emitted or transmitted by the search unit or a separate pitch search unit, provides an indication of the location of the particular acoustic discontinuity.
In addition, the orientation of the piezoelectric element with respect to the surface of the material to be inspected determines both the direction of ultrasonic wave propagation in the material and the mode of wave propagation. When the piezoelectric element is parallel to the surface of the material to be inspected, that is, when the element is situated so that its planar faces are perpendicular or normal to the surface, the piezoelectric element produces ultrasonic waves in the material in the longitudinal mode. In the longitudinal mode, the particle motion in the host material is parallel to the direction of wave propagation. When the piezoelectric element is inclined at an angle to the surface of the material to be inspected greater than zero but less than the first critical angle, the element produces ultrasonic waves in the material in both the longitudinal and shear wave mode by the mode conversion process. Alternatively, when the piezoelectric element is inclined above the first critical angle with respect to the surface of the material to be inspected, the element produces by mode conversion shear mode waves travelling at a certain angle in the material with respect to the material surface. In particular, a piezoelectric transducer element inclined in a search unit at an angle greater than the first critical angle with respect to the surface of the material to be inspected completely suppresses the creation of longitudinal mode waves in the host material by mode conversion, leaving only shear mode waves propagating in the material. In the shear mode, the particle motion in the host material is transverse to the direction of wave propagation.
Ultrasonic inspection or testing techniques are used for detecting material flaws in a number of situations and are particularly valuable in detecting flaws in the material of turbine or generator rotors. Turbine and generator rotors containing certain types of flaws in their material may fail abruptly and catastrophically, particularly under the start-up rotational and thermal stresses. The flaws in the rotor material may grow over the course of the many start-up sequences and finally link up with each other to form cracks, some perhaps capable of unstable growth under stress. Cracks with dimensions greater than the critical size for unstable growth may cause the rotor to burst or fracture under rotational and thermal stresses present during a final start-up sequence of the rotor.
Since the risk of failure depends upon a number of characteristics of the particular flaws, any useful rotor inspection must not only detect and pinpoint the location of flaws but also provide information on flaw type, size, and orientation. U.S. Pat. No. 3,960,006 to Smith shows one apparatus for performing ultrasonic inspections of turbine or generator rotors. The device included a transducer carriage for positioning a number of transmitting and receiving transducers within the rotor bore, a mechanical system for keeping track of the positions of the transducers, and a recording system. Smith utilized multiple ultrasonic signals in different modes of wave propagation (i.e. longitudinal, shear, and surface wave modes) in an attempt to characterize flaws in the rotor mass. Shear mode waves were directed axially or circumferentially at various angles into the mass of the rotor to perform pulse/echo inspections. Also, longitudinal waves were directed generally normal to the rotor bore surface and detected by a separate transducer adjacent to the transmitting transducer to provide a pitch/catch inspection.
The longitudinal wave pitch/catch inspection device taught by Smith did not, however, provide an effective inspection of the near-bore rotor material. This inability to provide an effective search in the near-bore region arose from two phenomena which Smith failed to recognize. First, leakage of the transmitted signal into the adjacent receiving or catch transducer presented a strong standing indication that obscured signals actually produced by any acoustic discontinuity in the near-bore region. Secondly, the complex near-field pressure distribution produced by the transmitting transducer extended into the near bore region of the rotor material and interfered with the energy reflected from the flaws in the near-bore region.
Even in regions distant from the near-bore region the Smith inspection device had a serious shortcoming. The Smith device relied on combining indications from different modes of wave propagation in an attempt to characterize flaws in the rotor material. However, the angular dependencies of the amplitudes of scattered and reflected ultrasonic waves are dependent upon mode type and frequency as well as upon actual reflector or flaw characteristics. In particular, the polar distributions of the amplitudes of ultrasonic waves reflected, scattered, and defracted from a material discontinuity will, in general, vary considerably with the particle motion direction, wave length, and frequency of the waves that impinge on the discontinuity. Frequency is a strong determiner of attenuation but it is also a determiner of wavelength for a specified mode type since each mode type has an associated wave propagation velocity in a specified material. Because the amplitude of scattered and reflected ultrasound is not uniquely related to flaw characteristics, the characterization of flaws using amplitude data from multiple wave modes, as proposed by Smith, did not necessarily produce valid results. Specifically, even where the longitudinal wave pitch/catch inspection produced flaw indications, the information in such longitudinal wave indications could not necessarily be compared with the information obtained from the shear wave pulse/echo indications to reliably and accurately characterize flaws with respect to type, orientation, dimensions, and other parameters.